Lesson 31 · The && operator

The && operator

The `&&` operator runs the next command only if the last one succeeded. Today you add your first conditional connector, the one behind every "build && test" you have ever typed.

The goal

Implement `&&` so the pipeline after it runs only when the previous pipeline's status was zero.

Start here - the target

TO DO

Scenario: Conditional AND

Giventhe line: true && echo yes

Whenthe shell runs it

Thenthe output is "yes\n" (the left side succeeded, so the right side ran)

And"false && echo yes" prints nothing (the left side failed, so the right side is skipped)

Background

&& is a conditional connector: it runs the pipeline to its right only if the pipeline to its left succeeded, meaning exited with status 0. It is how you say “do this, and only if that worked, do the next thing” - mkdir build && cd build, make && make test. If the left side fails, the right side is skipped entirely and the failure status carries forward.

Slot it into the connector loop from yesterday. Where ; always runs the next pipeline, && consults the last status first and runs the next pipeline only when it is zero. This “stop early on failure” behavior is called short-circuit evaluation, and it is the same idea as && in most programming languages - except here each operand is a whole command whose success is its exit status.

Make it work

// after running the left pipeline, check last_status before the right one
run_pipeline(left);
if (connector == AND && last_status == 0)
    run_pipeline(right);       // only on success

CheckpointDONE

`&&` chains commands on success. Commit and stop here.